What does the radius vector do on an SSS shader?
From the Blender Internal Wiki:

RGB Radius:
This is not in fact the radius of the subsurface scattering, but the average path length between scattering events. As the light travels through the object it bounces around then emerges from the surface at some other point. This value corresponds to the average length the light travels between each bounce. The longer the path length is, the further the light is allowed to scatter.
This is the main source of a material's perceived &quot;scatter color.&quot; A material like skin will have a higher red radius than green and blue. Subsurface scattering is the diffusion of light beneath the surface. You control how far the light spreads to achieve a specific result.

Basically you change the radius value for each of the three primary colors, which results in the efects you described in your question. In both render engines the Radius value does fundamentally the same thing.
The Cycles wiki states:

Radius input:
Scattering radius for each RGB color channel, the maximum distance that light can scatter.

This is fundamentally the same as the Blender Internal engine.
View the entire entry on the Blender Internal SS Shader.
View the entire entry on the Cycles SS shader.